Differential distributions of red–green and
blue–yellow cone opponency across the visual field

Abstract

The color vision of Old World primates and humans uses two
cone-opponent systems; one differences the outputs of L and M
cones forming a red–green (RG) system, and the other
differences S cones with a combination of L and M cones forming
a blue–yellow (BY) system. In this paper, we show that
in human vision these two systems have a differential distribution
across the visual field. Cone contrast sensitivities for sine-wave
grating stimuli (smoothly enveloped in space and time) were
measured for the two color systems (RG & BY) and the achromatic
(Ach) system at a range of eccentricities in the nasal field
(0–25 deg). We spatially scaled our stimuli independently
for each system (RG, BY, & Ach) in order to activate that
system optimally at each eccentricity. This controlled for any
differential variations in spatial scale with eccentricity and
provided a comparison between the three systems under equivalent
conditions. We find that while red–green cone opponency
has a steep decline away from the fovea, the loss in
blue–yellow cone opponency is more gradual, showing a
similar loss to that found for achromatic vision. Thus only
red–green opponency, and not blue–yellow opponency,
can be considered a foveal specialization of primate vision
with an overrepresentation at the fovea. In addition, statistical
calculations of the level of chance cone opponency in the two
systems indicate that selective S cone connections to
postreceptoral neurons are essential to maintain peripheral
blue–yellow sensitivity in human vision. In the
red–green system, an assumption of cone selectivity is
not required to account for losses in peripheral sensitivity.
Overall, these results provide behavioral evidence for functionally
distinct neuro-architectural origins of the two color systems
in human vision, supporting recent physiological results in
primates.